Oligonucleotides are known to hybridize to single-stranded RNA or single-stranded DNA. Hybridization is the sequence-specific base pair hydrogen bonding of bases of the oligonucleotides to bases of target RNA or DNA. Such base pairs are said to be complementary to one another.
In determining the extent of hybridization of an oligonucleotide to a complementary nucleic acid, the relative ability of an oligonucleotide to bind to the complementary nucleic acid may be compared by determining the melting temperature of a particular hybridization complex. The melting temperature (T.sub.m), a characteristic physical property of double helices, denotes the temperature (.degree.C.) at which 50% helical (hybridized) versus coil (unhybridized) forms are present. T.sub.m is measured by using the UV spectrum to determine the formation and breakdown (melting) of the hybridization complex. Base stacking which occurs during hybridization, is accompanied by a reduction in UV absorption (hypochromicity). Consequently, a reduction in UV absorption indicates a higher T.sub.m. The higher the T.sub.m, the greater the strength of the bonds between the strands.
Oligonucleotides can be used to effect enzymatic cleavage of a target RNA by using the intracellular enzyme RNase H. The mechanism of such RNase H cleavage requires that a 2'-deoxyribofuranosyl oligonucleotide hybridize to a target RNA. The resulting DNA-RNA duplex activates the RNase H enzyme and the activated enzyme cleaves the RNA strand. Cleavage of the RNA strand destroys the normal function of the target RNA. Phosphorothioate oligonucleotides operate via this type of mechanism. However, for a DNA oligonucleotide to be useful for cellular activation of RNase H, the oligonucleotide must be reasonably stable to nucleases in order to survive in a cell for a time period sufficient for RNase H activation. For non-cellular uses, such as use of oligonucleotides as research reagents, such nuclease stability may not be necessary.
Several publications of Walder et al. describe the interaction of RNase H and oligonucleotides. Of particular interest are: (1) Dagle et al., Nucleic Acids Research 1990, 18, 4751; (2) Dagle et al., Antisense Research And Development 1991, 1, 11; (3) Eder et al., J. Biol. Chem. 1991, 266, 6472; and (4) Dagle et al., Nucleic Acids Research 1991, 19, 1805. According to these publications, DNA oligonucleotides having both unmodified phosphodiester internucleoside linkages and modified phosphorothioate internucleoside linkages are substrates for cellular RNase H. Since they are substrates, they activate the cleavage of target RNA by RNase H. However, the authors further note that in Xenopus embryos, both phosphodiester linkages and phosphorothioate linkages are also subject to exonuclease degradation. Such nuclease degradation is detrimental since it rapidly depletes the oligonucleotide available for RNase H activation.
As described in references (1), (2) and (4), to stabilize oligonucleotides against nuclease degradation while still providing for RNase H activation, 2'-deoxy oligonucleotides having a short section of phosphodiester linked nucleotides positioned between sections of phosphoramidate, alkyl phosphonate or phosphotriester linkages were constructed. While the phosphoamidate-containing oligonucleotides were stabilized against exonucleases, in reference (4) the authors noted that each phosphoramidate linkage resulted in a loss of 1.6.degree. C. in the measured T.sub.m value of the phosphoramidate containing oligonucleotides. Such a decrease in the T.sub.m value is indicative of a decrease in hybridization between the oligonucleotide and its target nucleic acid strand.
Applications of oligonucleotides as diagnostics, research reagents, and therapeutic agents require that the oligonucleotides be transported across cell membranes or taken up by cells, appropriately hybridize to target RNA or DNA, and subsequently terminate or disrupt nucleic acid function. These critical functions depend partly on the initial stability of oligonucleotides towards nuclease degradation. Further, these functions depend on specificity of the oligonucleotide for a target DNA or RNA molecule.
A serious deficiency of oligonucleotides for these purposes is their susceptibility to enzymatic degradation by a variety of ubiquitous nucleases which may be intracellularly and extracellularly located. Unmodified, "wild type", oligonucleotides are not useful as therapeutic agents because they are rapidly degraded by nucleases. Therefore, modification of oligonucleotides for conferring nuclease resistance on them has been the primary focus of research directed towards the development of oligonucleotide therapeutics and diagnostics.
Modifications of oligonucleotides to enhance nuclease resistance has generally taken place on the sugar-phosphate backbone, particularly on the phosphorous atom. Phosphorothioates have been reported to exhibit resistance to nucleases. In addition, phosphorothioate oligonucleotides are generally more chemically stable than natural phosphodiester oligonucleotides. Phosphorothioate oligonucleotides also exhibit solubility in aqueous media. Further, phosphorothioate oligonucleotide-RNA heteroduplexes can serve as substrates for endogenous RNase H. Additionally, phosphorothioate oligonucleotides exhibit high thermodynamic stability. However, while the ability of an oligonucleotide to bind to a target DNA or RNA with fidelity is critical for its hybridization to the target DNA or RNA, modifications at the phosphorous atom of the oligonucleotides, while exhibiting various degrees of nuclease resistance, have generally suffered from inferior hybridization properties [Cohen, J. S., Ed., Oligonucleotides: Antisense Inhibitors of Gene Expression (CRC Press, Inc., Boca Raton, Fla., 1989].
One reason for this inferior hybridization may be the prochiral nature of the phosphorous atom. Modifications on the internal phosphorous atom of modified phosphorous oligonucleotides results in Rp and Sp stereoisomers. Modified phosphorus oligonucleotides obtained thus far, wherein the resulting molecule has nonsymmetrical substituents, have been racemic mixtures having 2.sup.n isomers, with n equal to the number of phosphorothioate intersugar linkages in the oligonucleotide. Thus, a 15-mer phosphorothioate oligonucleotide, containing 14 asymmetric centers has 2.sup.14 or 16,384 diastereomers. In view of this, in a racemic mixture, only a small percentage of the oligonucleotides are likely to specifically hybridize to a target mRNA or DNA with sufficient affinity.
Chemically synthesized phosphorothioate oligonucleotides having chirally pure intersugar linkages had thus far been limited to molecules having only one or two diastereomeric intersugar linkages. Until recently, the effects of induced chirality in chemically synthesized racemic mixtures of sequence-specific phosphorothioate oligonucleotides had not been assessed since synthesis of oligonucleotides having chirally pure intersugar linkages had yet to be accomplished by automated synthesis. This was due to the non-stereospecific incorporation of sulfur during automated synthesis. For example, Stec et al., J. Chromatography, 326: 263 (1985), synthesized certain oligonucleotide phosphorothioates having racemic intersugar linkages, however, they were able to resolve only the diastereomers of certain small oligomers having one or, at most, two diastereomeric phosphorous intersugar linkages.
However, Stec et al. [Nucleic Acids Res., 19: 5883 (1991)] subsequently reported the automated stereocontrolled synthesis of oligonucleotides. The procedure described in the above-mentioned reference utilizes base-catalyzed nucleophilic substitution at a pentavalent phosphorothioyl center.
The synthesis of phosphorothioates having all Rp intersugar linkages using enzymatic methods has been investigated by several authors [Burgers and Eckstein, J. Biological Chemistry, 254: 6889 (1979); Gupta et al. , J. Biol. Chem., 256: 7689 (1982); Brody and Frey, Biochemistry, 20: 1245 (1981); and Eckstein and Jovin, Biochemistry, 2: 4546 (1983)]. Brody et al. [Biochemistry, 21: 2570 (1982)] and Romaniuk and Eckstein, [J. Biol. Chem., 257: 7684 (1982)] enzymatically synthesized poly TpA and poly ApT phosphorothioates, while Burgers and Eckstein [Proc. Natl. Acad. Sci. U.S.A., 75: 4798 (1978)] enzymatically synthesized poly UpA phosphorothioates. Cruse et al. [J. Mol. Biol., 192: 891 (1986)] linked three diastereomeric Rp GpC phosphorothioate dimers via natural phosphodiester bonds into a hexamer.
The relative ability of an oligonucleotide to bind to complementary nucleic acids may be compared by determining the melting temperature of a particular hybridization complex. The melting temperature (T.sub.m), a characteristic physical property of double helixes, denotes the temperature (.degree.C.) at which 50% helical versus coil (unhybridized) forms are present. T.sub.m is measured by using the UV spectrum to determine the formation and breakdown (melting) of hybridization. Base stacking which occurs during hybridization, is accompanied by a reduction in UV absorption (hypochromicity). Consequently a reduction in UV absorption indicates a higher T.sub.m. The higher the T.sub.m, the greater the strength of the binding of the strands. Non-Watson-Crick base pairing has a strong destabilizing effect on the T.sub.m.
In a preliminary report [Stec, J. W., Oligonucleotides as Antisense Inhibitors of Gene Expression: Therapeutic Implications, Meeting abstracts, Jun. 18-21, 1989], thymidine homopolymer octamers having all but one linkage being modified phosphate linkages ("all except one") Rp stereoconfiguration or "all except one" Sp stereoconfiguration in the intersugar linkages were formed from two thymidine methylphosphonate tetrameric diastereomers linked by a natural phosphodiester bond. It was noted that a Rp "all except one" methylphosphonate non-sequence-specific thymidine homooctamer, i.e. (dT).sub.8 having all but one Rp intersugar linkage, formed a thermodynamically more stable hybrid (Tm 38.degree. C.) with a 15-mer deoxyadenosine homopolymer, i.e. (dA).sub.15, than a hybrid formed by a similar thymidine homopolymer having "all except one" Sp configuration methylphosphonate linkages and of d(A).sub.15 (Tm &lt;0.degree. C.), i.e. a d(T).sub.15 having all but one Sp intersugar linkage. A hybrid between (dT).sub.8 having natural phosphodiester linkages, i.e. octathymidylic acid, and d(A).sub.15 was reported to have a Tm of 14.degree. C.
More recently, Ueda et al. [Nucleic Acids Research, 19: 547 (1991)] enzymatically synthesized mRNAs intermittently incorporating Rp diastereomeric phosphorothioate linkages for use in translation systems. Ueda et al. employed T7 coliphane DNA having seventeen promoters and one termination site for T7 RNA polymerase. In vitro synthesis by T7 RNA polymerase produced mRNAs having from several hundred to tens of thousands of nucleotides.
Backbone chirality may also affect the susceptibility of a phosphorothioate oligonucleotide-RNA heteroduplex to RNase H activity. The ability to serve as a template for RNAse H has significant therapeutic implications since it has been suggested that RNAse H causes cleavage of the RNA component in an RNA-DNA oligonucleotide heteroduplex. With oligonucleotides containing racemic mixtures of Rp and Sp intersugar linkages, it is not known if all phosphorothioate oligonucleotides can function equally as substrates for RNase H. For a variety of catalytic reactions, hydrolysis of the phosphodiester backbone of nucleic acids proceeds by a stereospecific mechanism (an in-line mechanism) and inversion of configuration. Therefore, there may be only a small percentage of oligonucleotides in a racemic mixture that contain the correct chirality for maximum hybridization efficiency and termination of translation. Thus, increasing the percentage of phosphorothioate oligonucleotides that can serve as substrates for RNAse H in a heteroduplex will likely lead to a more efficacious compound for antisense therapy.
To enhance hybridization fidelty, phosphorothioate oligonucleotides having substantially chirally pure intersugar linkages are greatly desired. Further, such phosphorothioate oligonucleotides having substantially chirally pure intersugar linkages would lead to more efficacious therapeutic compounds. However, until now little success has been achieved in synthesizing such molecules. Therefore, simple methods of synthesizing phosphorothioate oligonucleotides having substantially chirally pure intersugar linkages are greatly desired.
It has been recognized that nuclease resistance of oligonucleotides and fidelity of hybridization are of great importance in the development of oligonucleotide therapeutics. Oligonucleotides possessing nuclease resistance are also desired as research reagents and diagnostic agents.